Graphene is a single layer of sp2 bonded carbon atoms arranged in a hexagonal benzene-ring structure and is as such essentially two-dimensional. It has, as late as in 2003, been found to be present in ordinary graphite. Graphene is in graphite stacked along the c-axis of the structure via weak van der Waal forces. The strong covalent in-plane bonding and the weak inter-planar bonding forces determine the anisotropic properties of graphite. The presence of graphene in graphite has been demonstrated by peeling off graphene sheets by a simple scotch tape technique from graphite.
Graphene has lately attracted considerable attention due to its unique properties, such as very high electron mobility and efficient heat dissipation, making it interesting especially for the microelectronic industry. It is considered to be a potential successor for silicon in the continuing desire to miniaturize and produce more efficient electronic components.
Graphene may be produced by solid state graphitization by decomposition or sublimation of silicon atoms from a silicon carbide surface. During this process, silicon leaves the surface as a vapor whereas carbon atoms stay as a residue on the silicon carbide surface. Under arbitrary process conditions, the quality of the carbon on the surface is typically equivalent to amorphous graphite and such a surface is therefore often called graphitized. However, when process conditions are properly selected, the desired ordered honey-comb structure of carbon atoms may be formed on the surface. It is believed that the graphitization starts at about 1150° C. in ultra high vacuum. However, in order to achieve graphene, the temperature must be much higher.
One example of such a process is disclosed in CN101602503A wherein a 4H-SiC (0001) surface is cleaned and smoothened by hydrogen and propane, respectively, followed by silane so as remove surface oxides. Thereafter, graphene is grown on the surface by evaporation of silicon at 1590-1610° C. and 890-910 mbar argon pressure for 30-60 minutes. The process proposed by CN101602503 requires the initial steps of cleaning and smoothening making it relatively complex and expensive, and therefore does not seem to be a commercially viable process.
Tzalenchuk et al., “Towards a quantum resistance standard based on epitaxial graphene”, Nature Nanotechnology, 5 (2010) 186, discloses that graphene can be grown epitaxially on silicon carbide. Graphene was grown at 2000° C. and 1 atm argon gas pressure, resulting in monolayers of graphene atomically uniform over more than 50 μm2. Tzalenchuk et al. further discloses that graphene was grown on the Si-face of silicon carbide since the reaction kinetics is slower there than on the C-face because of higher surface energy, and that this fact aids in the control of the formation of homogenous graphene.
Other processes for production of graphene includes for example carbonizing a precursor polymer as disclosed in U.S. Pat. No. 7,071,258 B1, and chemical vapour deposition as disclosed in WO 2009/119641.
There is however still some fundamental obstacles to overcome before graphene based materials can reach their full potential and be commercially successful. For example, the previously mentioned processes are impractical for large-scale manufacturing as they tend to result in graphene layers which are not homogenous, layers which comprise grains or defects, and/or layers which suffer from strong variation in carrier density across the layers grown.